Fixing PropTypes warning messages (#2670)

* Fixing PropTypes warning message

React recently started warning on the upcoming deprecation of
React.PropTypes, the new approach is to use the `prop-types`
npm package instead.

* Fixing the tests

superset/assets/javascripts/SqlLab/components/Link.jsx

@@ -1,14 +1,15 @@
 import React from 'react';
+import PropTypes from 'prop-types';
 import { OverlayTrigger, Tooltip } from 'react-bootstrap';
 
 const propTypes = {
-  children: React.PropTypes.node,
-  className: React.PropTypes.string,
-  href: React.PropTypes.string,
-  onClick: React.PropTypes.func,
-  placement: React.PropTypes.string,
-  style: React.PropTypes.object,
-  tooltip: React.PropTypes.string,
+  children: PropTypes.node,
+  className: PropTypes.string,
+  href: PropTypes.string,
+  onClick: PropTypes.func,
+  placement: PropTypes.string,
+  style: PropTypes.object,
+  tooltip: PropTypes.string,
 };
 const defaultProps = {
   className: '',